In recent years, a server which provides a game to portable devices via a communication network has been commonly used. The game provided by the server includes one in which multiple users can participate (i.e., a “social game”), in which the users cannot only battle and cooperate with each other but also communicate with each other (for example, refer to Japanese Unexamined Patent Publication No. 2005-034303).
As an example of such a social game, a game is known in which a user battles against an enemy character by using, for example, cards (i.e., a “card battle game”) (for example, refer to Japanese Unexamined Patent Publication No. 2012-061059). In a card battle game, by using cards, a user battles against an enemy character created by the game program, while cooperating with a different user. Each user can possess an arbitrary number of cards. Further, by using at least one possessed card, each user forms a set of cards (i.e., a “deck”) to be used for the battle. Each card has parameters such as attack power, defense power, and health point, and the users reduce the health point parameter value of the enemy character based on the parameter values of the cards which form the deck. The enemy character also has parameters similarly to the cards, and, based on these parameter values, reduces the health point parameter values of the cards. The users and the enemy character alternately attack each other, and the battle ends when the health point parameter value of the cards which form the deck has fallen to 0, or when the health point parameter value of the enemy character has fallen to 0.